Pyroclastic density currents and the sedimentation of ignimbrites

by
Michael Branney1 and Peter Kokelaar2

1Geology Department, University of Leicester, University Road, Leicester, LE1 7RH, UK
2Earth Sciences Department, Liverpool University, Liverpool, L69 3BX, UK

Geological Society of London, Memoirs, 27, pp 152. (5 tables, 38 diagrams, 95 photographs)
ISBN 1-86239-124-6 Paperback.
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Contents
Preface vii
Acknowledgements viii

Chapter 1 Introduction and key concepts 1
Deposits of pyroclastic density currents 1
The role of ignimbrites in ideas about pyroclastic density currents 1
Key concepts 2
Current steadiness and uniformity 2
Lower flow-boundary zones: sites of segregation and variable deposition 4
Ignimbrite architecture: a record of flow-boundary zone evolution through time and space 4
Chapter 2 The origin, nature and behaviour of pyroclastic density currents 7
Origin and development of pyroclastic density currents 7
Eruption styles 7
Current concentration and rheology 8
Deflation reappraised 10
The nature of pyroclastic density currents 10
The leading part of the current 10
Current velocity 11
Velocity profiles and turbulence intensity 11 Plug flow 13
Current stratification 14
Partitioning of mass flux in density-stratified currents 15
The behaviour of pyroclastic density currents 16
Inertia, buoyancy, runout distance and lofting 16
Internal waves, hydraulic jumps and granular jumps 18
Thalwegs (flow axes) and lateral migration 18
Effects of topography 18
A new two-fold classification of pyroclastic density currents 20
Chapter 3 Mechanisms of particle support and segregation 23
Significance of current heterogeneity and pyroclast diversity 23
Fluid turbulence 24
Support by fluid turbulence 24
Vertical segregation of clasts in the current during turbulent transport 24
Segregation at the flow-boundary zone due to turbulence 24
Support on an interface 25
Sustained support: rolling and sliding 25
Intermittent support: saltation 25
Overpassing: downcurrent segregation at an interface 28
Granular temperature and dispersive pressure 29
Clast interactions and current mobility 29
Segregation in granular flows 29
Fluidization 31
Fluidization and transport 31
Segregation by fluidization 33
Hindered settling, fluid-escape and sedimentation-fluidization 33
Hindered settling and current mobility 34
Segregation by fluid-escape and hindered settling 34
Clast buoyancy 34
Buoyancy and transport 34
Segregation by buoyancy 34
Acoustic mobilisation 35
Acoustic mobilisation and transport 35
Segregation by acoustic mobilisation 35
Support by strength 35
Quasi-static grain contacts 35
Cohesion 35
Segregation associated with strength 35
Particle interlocking 35
Chapter 4 Conceptualising deposition: a flow-boundary zone approach 37
Deposition from steady currents 37
Direct fallout-dominated flow-boundary zone 37
Traction-dominated flow-boundary zone 37
Granular flow-dominated flow-boundary zone 39
Fluid escape-dominated flow-boundary zone 39
Gradational types of flow-boundary zone 41
Selective filtering: flow-boundary zone segregation and overpassing during deposition
41
Traction carpets 42
Deposition during unsteadiness 43
Fluctuating deposition 43
Sustained gradual changes 43
Rapid deposition 45
En masse deposition 45
Non-uniform deposition 47
Interpreting ignimbrite lobes and levees 47
Post-depositional remobilisation 49
Effects of deposition on current behaviour 49
Chapter 5 Interpreting ignimbrite lithofacies 51
A lithofacies scheme for ignimbrites 51
Massive lapilli-tuff lithofacies 51
Description 51
Interpretation 56
Origin of fabrics in massive lapilli-tuff lithofacies 56
Massive to stratified lithic breccia lithofacies 57
Description 57
Interpretation 57
Segregation of blocks from pumice and ash 60
Interpretation of stratified breccias 60
Classifications of ignimbrite breccias 6
Massive agglomerate lithofacies 61
Description 61
Interpretation 61
Lithofacies with fines-poor (elutriation) pipes, sheets or pods 61
Description 61
Interpretation 66
Vertical grading patterns 66
Description 66
Interpretation 66
Diffuse-stratified and thin-bedded lithofacies 71
Description 71
Interpretation 71
Stratified and cross-stratified tuffs 74
Description 74
Interpretation 74
Pumice-rich layers, lenses and pods 76
Description 76
Interpretation 76
Massive and parallel-bedded lapilli layers 77
Description 77
Interpretation 77
Parallel-bedded and parallel-laminated tuff 83
Description 83
Interpretation 83
Eutaxitic, rheomorphic and lava-like lithofacies 83
Description 83
Interpretation 83
Origin of poor sorting in ignimbrites 84
Chapter 6 Ignimbrite architecture: constraints on current dynamics 87
Conceptualising architecture in a time-geometry framework 87
Longitudinal architectures 90
Transverse architectures 91
Interpreting longitudinal (proximal to distal) lithofacies variations 91
Longitudinal coarse-tail grading 91
Downcurrent lithofacies changes from stratified to massive 91
Distal lithofacies changes from massive to stratified 93
Interpreting vertical lithofacies variations 95
Gradational versus sharp lithofacies variations 95
Bedding and flow-unit boundaries 95
Repetitious and rhythmic lithofacies successions 98
Disordered lithofacies successions 98
Intercalated massive and stratified divisions 98
Complex longitudinal architectures 98
Interpreting lithofacies successions at bases of ignimbrites 99
Stratified bases 99
Basal pumice lenses 101
Fines-poor bases 101
Fine-grained layers at the base of ignimbrites 101
Some common successions of basal lithofacies 108
Sheared or loaded substrate 108
Interpreting lithofacies at the top of ignimbrites 109
Transverse lithofacies variations 109
Splay-and-fade stratification 109
Scour splay-and-fade stratification 109
Effects of current thalwegs and braiding 111
Gradations between massive valley-filling ignimbrite and stratified topographic veneers
111
Radially symmetrical ignimbrite distributions 113
Ignimbrite fans and asymmetric ignimbrite distributions 115
Interpreting the shape of ignimbrites 115
A classification of ignimbrite shape 115
Significance of aspect ratio 117
Top surfaces of ignimbrites 118
Chapter 7 Overview, key implications and future research 119
Overview and key implications 119
Future research 121
Definitions of terms used 123
References 127
Index 137

Preface

Ignimbrites are vast, landscape-modifying deposits composed mainly of pumice fragments and ash. They derive from the most hazardous types of explosive volcanic eruptions and record rapid sedimentation from catastrophic pyroclastic density currents that sweep across the ground. Since early work on ignimbrites by P. Marshall (1945), H. Kuno (1941), R.L. Smith (1960), and R.V. Fisher (1966), there has been a dramatic increase in research into these enigmatic deposits. Particularly instructive field studies include those of ignimbrites from the large caldera volcanoes of the western USA, from the arc volcanoes of the Mediterranean region, Japan, Southeast Asia, South America and New Zealand, and from intraplate volcanoes such as the Canary islands. Experimental-analogue and numerical modelling of pyroclastic density current behaviour and sedimentation have recently complemented the field-based work. Now there is a bewildering plethora of ignimbrite classification schemes, emplacement models, and deposit interpretations. It is therefore timely to take stock, to synthesise modern understanding, and, in particular, to consider how field investigations of ignimbrite lithofacies can best be used both to infer actual pyroclastic density current behaviour and to constrain or test the various models. A fresh look at ignimbrite emplacement is all the more important with the recognition that many ignimbrites can relate to eruptions with magnitudes sufficient to impact global climate and biota.
This Memoir reviews what is known about pyroclastic density currents and presents a new conceptual framework for investigating the deposition of all types of ignimbrite lithofacies. After introducing some key concepts in Chapter 1, we review important observations and experiments that bear on the nature and behaviour of pyroclastic density currents (Chapter 2), and on the mechanisms by which diverse particles are supported and variously segregated within them (Chapter 3). In Chapter 4 we present the conceptual framework that we have devised to comprehend how different ignimbrite lithofacies are deposited. In this framework, ignimbrite sedimentation is treated as a sustained flow-boundary process in which the sorting and bed-form characteristics of the deposit relate to different types of concentration and shear distributions within the flow-boundary zone that spans the basal part of the current and the uppermost part of the aggrading deposit. Chapter 5 describes and illustrates a wide variety of common ignimbrite lithofacies, including examples from around the world, and in it we apply the flow-boundary zone approach to provide some insights into how they may have formed. In Chapter 6, we elaborate the paradigm developed in earlier chapters to consider how the various architectures of ignimbrites may be used to reveal how flow-boundary zones of sustained currents evolved through time and space. We consider the diverse vertical and lateral lithofacies sequences exhibited by ignimbrites with reference to a temporal framework provided by time-surfaces called entrachrons and depochrons. Such sheet-scale analysis is important because an individual lithofacies provides information primarily only about the local flow-boundary zone, whereas the properties and behaviour of the current as a whole may only be deduced when the sheet-scale depositional history is understood.
The flow-boundary zone approach to interpreting ignimbrite sedimentation, linked with the scheme for analysis of ignimbrite lithofacies architecture, provides a powerful means to constrain the overall behaviour and evolution of unseen pyroclastic density currents. The approach begs further research into the mechanisms and rates of the various processes that are inferred. It also has applications for the interpretation of deposits from lahars, turbidity currents, and other types of granular, liquefied or fluidized sediment gravity flows. We hope that this Memoir both stimulates and facilitates further research into pyroclastic density current deposits and into experimental quantification of physical conditions and process rates.

KEY WORDS: density current, sedimentation, ignimbrite, pyroclastic flow, pyroclastic surge, granular flow, fluidization, hindered settling, granular segregation.

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